1. Field of the Invention
The present invention relates to an all-solid-state electric double layer capacitor, and particularly to improvement of a solid electrolyte.
2. Description of the Related Art
An electric double layer capacitor is a high-capacity capacitor in which an electric double layer is formed at an interface between an electrode and an electrolyte (electrolytic solution). Therefore, the electric double layer capacitor is used as a power source for backing up a PC power source, an in-vehicle power source, and the like by charges stored in the electric double layer capacitor upon temporary blackout of the power source. The electric double layer capacitor has a feature that it has high capacity and is capable of charging and discharging at high speed, and also causes less deterioration of performance even if charging and discharging are repeated.
With high-functionalization and miniaturization of PCs, there has recently been a demand for an electric double layer capacitor which can increase the capacity and also causes no leakage of an electrolytic solution providing high reliability. In order to obtain such an electric double layer capacitor, various studies have mainly been made on an electrolyte.
For example, Japanese Unexamined Patent Publication No. 2000-195759 proposes an electric double layer capacitor in which an electrolyte comprises a sulfonium salt having three alkyl groups of 1 to 6 carbon atoms. The patent document describes that, by using activated carbon as an electrode, propylene carbonate as a solvent and an electrolyte having a small ionic diameter such as triethylsulfonium tetrafluoroborate as the sulfonium salt, it becomes possible to insert a lot of the ions into pores of the activated carbon as the electrode, and thus a high-capacity electric double layer capacitor can be provided.
However, in the electric double layer capacitor as disclosed in Japanese Unexamined Patent Publication No. 2000-195759, an electrolytic solution is used and therefore deterioration and failure due to leakage of the electrolytic solution may arise.
Also, Japanese Unexamined Patent Publication No. 2002-324734 discloses a gel-like electrolyte for an aluminum electrolytic capacitor and a method for producing the same. The patent document proposes a composition forming the gel-like electrolyte, which is produced by radical polymerization of a polymerizable monomer using t-butylperoxybenzoate as an initiator, while aluminum is used as a current collector, imidazolium phthalate as an electrolyte and γ-butyrolactone as a solvent. The patent document describes that an electric double layer capacitor having high retention of an electrolytic solution and high heat resistance can be provided by using the gel-like electrolyte having high ionic conductivity.
However, in the capacitor using the gel-like electrolyte as disclosed in Japanese Unexamined Patent Publication No. 2002-324734, since the decomposition temperature of the gel-like electrolyte is usually about 250° C., the temperature range for operating this capacitor is limited to the range lower than the decomposition temperature of this gel-like electrolyte.
Furthermore, Japanese Unexamined Patent Publication No. 2004-067504 discloses a ceramic capacitor using barium titanate having a coated surface. The patent document proposes a laminated ceramic capacitor produced by forming barium titanate coated with a metal oxide, a metal hydrous oxide, a metal hydroxide or an organic acid salt of a metal other than barium or titanium into a green sheet, by alternately laminating the green sheet with a conductive agent and by sintering the laminate. The patent document describes that, by using barium titanate having the coated surface as a dielectric, short-circuiting becomes less likely to occur even if the thickness of the dielectric is decreased, thus making it possible to supply a high-capacity capacitor.
However, in the case of the ceramic capacitor disclosed in Japanese Unexamined Patent Publication No. 2004-067504, the thickness of the dielectric layer is decreased and therefore dielectric breakdown becomes more likely to occur. Also, a process for coating barium titanate is required and the sintering temperature of barium titanate itself is a high temperature of about 1,100 to 1,200° C., resulting in a high process cost.